Preview shortname if course custom field empty (#579)

Previously if customcertelement_coursefield was a course custom field
and this field had no value then it did not appear on the reposition
page and so could not be dragged to the required location.  Now when
this field has no value its shortname is shown instead.

element/coursefield/classes/element.php

@@ -138,7 +138,10 @@ protected function get_course_field_value(\stdClass $course, bool $preview) : st
         if (is_number($field)) { // Must be a custom course profile field.
             $handler = \core_course\customfield\course_handler::create();
             $data = $handler->get_instance_data($course->id, true);
-            if (!empty($data[$field])) {
+            if ($preview && empty($data[$field]->export_value())) {
+                $fields = $handler->get_fields();
+                $value = $fields[$field]->get('shortname');
+            } else if (!empty($data[$field])) {
                 $value = $data[$field]->export_value();
             }